1. Field of the Invention
The present invention relates to a digital audio broadcasting xe2x80x9cDABxe2x80x9d receiver.
2. Description of the Related Art
In Europe, DAB (digital audio broadcasting) according to the EUREKA 147 standard is now being practiced. In the DAB, a plurality of digital data are subjected to various encoding processes and finally converted to an OFDM (orthogonal frequency division multiplex) signal and the OFDM signal is transmitted. Digital data, such as digital audio data, of 64 channels (the maximum case) can be transmitted simultaneously.
FIG. 1A shows the structure of the OFDM signal on the time axis, which includes a plurality of frames that are arranged continuously. The DAB has four transmission modes, and FIG. 1A shows the structure of mode I in which one frame has a duration of 96 ms. Further, in mode I, each frame consists of 77 symbols. In each frame, the two head symbols constitute a sync channel and the subsequent three symbols constitute four fast information channels FIC, and the remaining 72 symbols constitute a main service channel MSC.
The sync channel is used to detect a frame phase and a deviation of the reception frequency (tuning frequency). The first symbol of the sync channel is a null symbol Null and the second symbol is a phase reference sync symbol TFPR. The null symbol Null in every other frame includes identification information TII for identifying a transmitting station, and nothing is transmitted in the interval of the null symbol Null in each of the remaining frames.
The fast information channels FIC serve to provide data relating to the main service channel MSC and other data. In mode I, each fast information channel FIC is divided into three fast information blocks FIB. Data relating to time, date, type, data arrangement, traffic message control, etc. are arranged in the fast information blocks FIB.
Digital audio data as main data and other various data are arranged in the main service channel MSC.
Further, as shown in FIG. 1B, each symbol has a structure that the head interval Ts having a predetermined duration is a guard interval as a guard in connection with the immediately preceding symbol and the remaining interval is an interval that is effective as a symbol. The contents of the interval Ts are made the same as the contents of a tail interval Te of the same symbol. Therefore, the intervals Ts and Te are the same also in duration. In mode I, the one symbol duration T=1,246 xcexcs and Ts=Te=246 xcexcs.
FIG. 2 shows the structure (also mode I) of the OFDM signal on the frequency axis. Many carrier components are distributed at regular intervals in the OFDM signal. In mode I, the number K of carrier components is 1,536 and the carrier spacing (frequency interval between adjacent carrier components) is 1 kHz. However, in any transmission mode, no carrier component exists at the position of the center frequency fc.
Transmission modes II-IV are different from mode I only in the frame duration and the size and the number of fast information channels FIC and fast information blocks FIB, and have the same basic structure as mode I. Therefore, modes II-IV will not be described any further. In the following description, specific values used are ones in mode I unless otherwise specified.
By the way, in mode I, since the carrier spacing of the OFDM signal is 1 kHz as described above, it is necessary that the reception frequency (tuning frequency) of the DAB receiver be locked in a certain frequency range (e.g., in this case, xc2x1500 Hz) with respect to the DAB broadcast frequency. To this end, it is necessary that the reception frequency be synchronized with (i.e., tuned to) the broadcast frequency by locking the local oscillation frequency of the DAB receiver at a frequency that is deviated from the broadcast frequency by the intermediate frequency.
In the DAB receiver, the FFT (fast Fourier transform) is used to decode received data. In the FFT, it is necessary that FFT windows be correctly located in time with respect to symbols. That is, it is necessary to take synchronization in time.
In view of the above, in the DAB receiver, time synchronization and tuning are made according to the following procedure:
(1) A null symbol Null is detected.
(2) A sync symbol TFPR ensuing the null symbol Null is captured by using a detection result of item (1).
(3) The captured sync symbol TFPR is subjected to FFT.
(4) A frequency error L as defined with the carrier spacing used as the unit and a frequency error xcex less than L/2 are determined based on an FFT result.
For example, in mode I, L=2 and xcex=0.3 if the reception frequency has an error 2.3 kHz.
(5) A time error is determined.
(6) The frequency and time errors are corrected by using results of items (4) and (5).
However, in the above method, processing relating to synchronization cannot be performed until a sync symbol TFPR is captured (item (2)). That is, the time required for items (1) and (2) is not a time necessary for the synchronization processing itself but is a wasteful time, and it elongates the time necessary for taking synchronization.
An object of the present invention is to substantially eliminate the time corresponding to the above items (1) and (2) and thereby shorten the time necessary for taking synchronization.
To attain the above object, the invention provides a digital audio broadcasting receiver comprising a delay circuit for delaying a received DAB signal by one symbol period minus a guard period; a correlation circuit for taking correlation between a delayed output of the delay circuit and the DAB signal; a moving average circuit for moving-averaging a correlation output of the correlation circuit over a width that is equal to the guard period; a peak detection circuit for detecting a position in time of a peak of a moving average output of the moving average circuit; a calculation circuit for calculating an error in a reception frequency based on a phase deviation in the DAB signal at the position in time that is indicated by a peak detection output of the peak detection circuit; an FFT circuit for subjecting the DAB signal to FFT; and a detection circuit for determining a reception center frequency based on an FFT output of the FFT circuit, wherein the error in the reception frequency is corrected by controlling a local oscillation frequency based on a calculation output of the calculation circuit; and wherein an error in the reception frequency is corrected by performing carrier shifting based on a detection output of the detection circuit.
Therefore, the time required for detecting a null symbol and the time required for capturing a sync symbol are not needed.